Political stances are mutable. I won't argue your point about whether she should be despised for the
rate of her mutability. I'm pointing out the flaw in your analogy. Someone taking a stance that contradicts a past stance is not in the same category as someone saying something that contradicts an observable/measurable quantity, unless you are in a possession of a mind-reading device and can observe Clinton's current belief regarding gay marriage.
Your analogy would work if you said liberals give Clinton a pass for flip-flopping while hating Trump for flip-flopping on abortion. AFAIK, liberals hate Trump for his current stance and actions on abortion rights, not for being a flip-flop (i.e. they'd be fine if the flip-flop happened in the other direction)